Instrumental sets by bands are usually just footnotes (see also the Beastie Boys' The Mix-Up). But this collection of dense headphone music feels like acreative breakthrough for Trent Reznor. A new-media showpiece like Radiohead's In Rainbows, Ghosts I-IV is a self-released kiss-off to Reznor's former label, Universal, which he ditched last year after months of public feuding. You can download part of it (free) or all 1.8 hours ($5), or order it in various physical configurations (the $300 limited edition has already sold out to fanatics). Musically, Ghosts rewards close attention. The touchstone is Brian Eno's headier work: the murky instrumentals on Another Green World, the angular rhythm collages of My Life in the Bush of Ghosts. But Reznor's own hyper-detailed language defines the set: heaving synthesizers, doleful piano, alien-insect noises. There are animalistic contributions by King Crimson guitarist Adrian Belew, so the apocalyptic ambience frequently rocks. And since it's licensed with Creative Commons (the organization that redefines copyright to encourage noncommercial artistic sampling), Ghosts' awesome sound palette may soon enter the digital-music lingua franca — a gift that should keep on giving.